Frequency modulation receiver



- Nov. 28, 1944.

M. G. cR'osBY FREQUENCY MODULATION RECEIVER F'iled Oct. 25, 1941 4 Sheets-Sheet l mal m w M M y vm Ira/ansie 070 7 AAAAA y lNvEN-roR 1g] C1 ginas/3g `AT'TCJRNEY MUT Nov. 28, 1944. M. G. CROSBY FREQUENCY MODULATION RECEIVER Filed Oct. 254, 1941 4 Sheets-Sheet 2 l l I l l :6'- l 7 l dat@ -Zaw env/de Nov. 28, 1944,.

M G. CROSBY 2,363,649

FREQUENCY MODULATION RECEIVER Filed oct. 25, 1941 4 sheets-sheet 5 .Rg I i 'VIVI' AT'TORNEY NOV. 28, 1944. `M G, CROSBY 2,363,649

FREQJENCY MODULATION RECEIVER lllll AA m A /I v INV-ENTOR' i Burz@ G. Cms/5g 4using the present invention,

Patented Nov. 28, 1944 FREQUENCY MoDuLA'rIoN RECEIVER `Murray G. Crosby Riverhead, N. Y.,` assignor `to Radio Corporation of America, acorporatonof.;

Delaware f Applicationoetober 25, 1941-, serial-No; 416,443' E (omen-2o) v 9 Claims.

My present invention relates generally to angular velocity-modulated carrier wave receivers, andimore particularly to a frequency modulation receiver wherein cancellation of` amplitude modulation effects is elected' without. a limiter and at a point following the balanced detector."

In the past undesired amplitude modulation veffects existing onl aniangular velocity-modulated carrier wave have been `removed by means* of a limiter, a balanced detector, or by a combination of both devices; By angularvelocityemodulated carrier wave is meant a phasaor frequency, modulated carrier wave. The expression ,"timing-modulated may, also, beused@ hereinafter generically to 'designate such types of modulation. ,v

The limiter is effective in removing amplitude Vmodulation atall times even in the presence of `frequency modulation, but. the balanced detector is only completely balanced for the condition of no-frequency` modulation present. Hence, when l the balanced detector is depended upon to remove the undesiredl amplitude modulation the elimina- `tion is. ,only complete in the absence of` the desired modulation, and becomes progressively poorer as the desired modulation is increased.

Basically, my presentinventioninvolves the utilization of detected amplitude `modulation existing ontheangular velocity-modulated car` rier wave to modulate: the detected modulated carrier wave energy with such polarity that the undesiredV amplitude modulationis greatly reduced. Hence, if desired, the limitermaybe dispensed with, since the elimination of lthe amplitude modulation by the balanced'detector is made more complete. By omitting the limiter the gain of the receiver may be decreased, since thelarge signal voltages required to saturate the `limiter do not have to be provided. Economy of construction andsimplificationfollow as a result of gain reductionv prior to lthedemodulator; Y

Thenovel features which. Ibelieveto be. characteristic of my invention are set forth with particularity in `the appended claims; the inventionitself, however, as to` both its organization and' method of operation. will `best be understood by reference to the following description taken in connection withy the drawings in which I have indicated diagrammatically'several circuit or.- ganizationslwhereby'my invention maybe carried Fig. 1A shows the rwave form of the detected output ofa frequency modulated carrier wave HPM), l f `4 l Fig. 1B illustrates the same wave Figs. 2 to 6 inclusive show respectivelydiiferent embodiments o'f the invention. L

form when Referring to the accompanying drawings, wherein likef'rei'erence characters in thediiferent figures designate similarcircuil'f,elements;` there willE first be` considered the manner in which my inverse modulation method eiect's` substantial suppression `of the amplitude modulation (AM) existing on the carrier. Fig, lill.` shows the manner in which the presence of amplitude modulae tion `is manifested in the-frequency modulation receiver demodulatoroutput'. Wave Ais thedesired detected` `frequencyv modulation `output `which is` assumed to' have a higherlfrequency than the undesired amplitude modulation frequency.A The undesired `amplitude modulation appears as an amplitude envelope `asshown by wave Bv of Fig. 1A. `The undesired' amplitude modulation is assumed to produce a 2lb-percent `modulation of theV incoming signal.

` In `the receiver ofthis invention, an amplitude 'modulation output is provided having `the wave form of vthe amplitude envelopeB. Thisoutput is used to modulated-.he` amplitude "of" wave A in a sense which causes Va reduction "of amplitude when the wave is at point Aafonth'e envelope, and

an increase in amplitude: when` they wave isV at point' t. The inverse modulationfthus applied, therefore, opposes thev undesired* amplitude modulation which ispresent, and tends tocancel it, Fig. 1B' `shows the result of"applying the inverse amplitude modulation `to the FM wave of Fig. 11A.` The resulting AM envelope? B" is of double frequency'and greatly reduced amplitude. Before evaluating the degree of reduction of AM effects possible bythe present reverse modulation process, one embodiment of the invention will be explained. In Fig; Zthereis shownonly so much `of an FM receiver as'f'is essential to Aa proper understan-ding of AtheA invention. `Let it be assumed that `the balanced detectors 2`3iare preceded' by the usual `pre-demodulatorstages of a superheterodyne receiveradapted` to operate in ing frequency. At present, a-*maximurn center frequency (Fel deviation of "l5V kilocycl'es (k'c.) is used f'or FM broadcasting; In the superhetero dyne` receiver the FMT `waves'` are reduced to an intermediate `frequency (T. Fil ,althoughthe` same IV frequency deviation range is` retained'. The FM waves, withn center frequency of the I. F. value, i

are tio-be understood' as applied tothe resonant application Serial No. 402,784, led'July 1'7,` 1941 l In a modulator of the type shown at tube 4 "the carrier is usually fed to the third grid Il",`

patented September 15,- 194:2,4 as U. S. Patent No.

2,296,092. Such a detection network provides balanced detected FM Voltage and a separate AM voltage. The anode of diode 2 is connected to itsV cathode through a series path comprising secondary circuit I', resistor R1 and resistor R2,

-the two resistors being shunted by the by-pass The anode of diode 3 is connectedl condenser C1. to its cathode through a series path comprising the secondary circuit I", resistor R3 and resistor R4, the Icy-pass condenser C2 shunting R3-R4.

Each of circuits I'and I is magnetically cou pled to tuned circuit 'I, but circuits I and I" are equally andoppositelymistuned relative to the I. F. value of circuitl I by any predetermined frequency separation. As explained in the aforesaid patent, detected FM voltage is developed across resistors R2 andA Ri which are arranged in polarity opposition, it being noted that the cathode `end of resistor R4 is at ground potential.

The detected YAM voltage may be taken oi at the anode end of resistor'Ri. lThis type of discriminator converts applied FM waves to AM waves with modulation envelopes 180 degrees out of phase. Thesev modulation envelopes are deand appears across a tuned circuit in the plate circuit. The modulation applied to the first grid is normally of a frequency such that the tuned circuit does not pass it. However, in this type of audio modulator a resistor Ill is employed inthe platecircuit, and there appears across the resistor in amplied form the detected AM voltage originally applied to grid 4. There, also, appears across the resistor I0 the inversely modulated detected FM voltage.

In other words, at tube 4 the detected AM voltage is applied to grid 4 in such a phase relation to the detected FM voltage at grid 4 that the wave form of the resultant inversely modulated, detected FM voltage is as shown in Fig. 1B. The inverse modulation causes a rise in the gain of modulator tube 4 when the undesired AM causes a reduction of the incoming signal ampli-A I ulator output by means of a balancing system tected by diodes 2 and 3 which make the detected outputs available across resistors Ri-Rz and `Rca-R4. The manner in which Ri-Rz and RQc-R4 are connected together makes the detected -FM voltage available across Ra-Rr.

v The detected AM voltage is taken from across resistors R1 and R4. It will be 'noted' that these two resistors R1 and R4 are connected so that they both have their ends which are towards the diode at high potential, and the combination of the detected yAM voltage outputs is in phase. The normal adjustments of the resistors are to make them all equal in-,order to effect balanced conditions for both AM and FM reception. By-pass condenserswill by-passkthe `I. F. currents fed to the discriminator. but allow the modulation frequencies to pass. There will, then. be derived from the upper end of R2 the detected FM voltagewhich, also, has superposed the detected AM eiects'which are to be eliminated. AFrom the anode end of Ri there is derived the detected AM voltage per se.

` The cathode end of R2 is connected to .ground through a series pathcomprising condenser Cz and resistor 6. Y An adjustable tap is providedto takeoff from the potentiometer 6 any desired magnitude of voltage thereacross. vIn the same way the series path C4-5 acts asa potentiometer connected between the `anode end of R14 and ground. The tap lslidalcle along resistor provides an adjustment for feeding a desired` magnitude of detected AM voltage'to tube Il. The latter acts as aI modulator tube, and may be of the pentagrid type. y

f It includes a cathode, a plate andflve successive control grids.: Grid 4" is connected tothe resistor I5, andV hasthe detected FM voltageap.- plied thereto.; Grid 4 is connected to resistor 5, and has the 'detected AM voltage impressed thereon. The plate of tube `I! is connected to a source of positive voltage, through load resistor I0. the screen grids on either side of grid 4 embodying tube 1. The latter; for example, may

IVbea twin triode tube. The `common cathode connection cf4 tube 1 is connected through a resistor 8 to the grounded end of potentiometer resistor 8. The upper end ofthe latter is connected to .the plate end of resistor `I0 by'a condenser Cs.

The grid I of one of the triodes of tube 'I is connected by an adjustable tap along resistor 8, while the grid 'I"`of the other triode is connected by lead I0' to the adjustable tap which feedsthe detected AM voltage to grid 4. Y The plates of tube I are connected in parallel vto a common source of positive direct current voltage through load resistor 9. y

As the detected AM voltage is ampliiied4 by the modulator tube 4 its phase is reversed. When combined in-phase, as is 'done by tube 1, bucking results. `Potentiometer 8 may be adjusted so that the undesired detected AM voltage transu'lation is considered. It is obvious that when A the amplitude modulation modulates the 'carrier to zero amplitude, itI would take an infinite amount of gain to bring it back to normal amplitude. Y

On the other hand,for the upward peaks of amplitude modulation the condition of per` cent upward modulation may be compensated for by a reduction in gain of 2:1.; Thus, the

upward and downward -swings of the modulationv do not follow a linear law which allows complete cancellation by a simple application of lreverse modulation. The degree of reductionhpos- .sible by this reverse modulationprocess will now Ibe evaluated in the following analysis:v

` detector is thus:

assaut and `also, an namplitude envelopej hence, vit isg'ivenby: f

'e= 'E01 -Fmsin pt) 'sin (ot-gl cos qt) (1) inwhich pi iis ythe amplitude modulation factor, pis the angular velocity fof the amplitude modulating Wave, o `is the lcarrier angular `velocity,\Fd is the peak-frequency devi-ation; Fm is the modulation frequency of theapplied frequency modu-` lation, `ando fis the angular velocity of the applied frequency modulation. This I signal is passed through `the two ysloping 4iilters, of the balanced @.:EoJfmSin patinLkFdlsi qt) sin (MTF-f; cos qt (2) where k `is ,a constant which is determined by the .steepness .of the sloping filters. When the Wavegiven by (2) is detected, the output is proportional to the amplitude envelope. The amplitude envelope consists of the two quantities in parentheses which multiply `outto give: ep-EK ici-Fmsmpti-k meinemi kFamsin kpt sin at)` (3) in which` K` is theconstant of proportionality of the` detectors. i

, 'Ihe second of thev two sloping filters `has ar slopewhich (is Opposite to `that 'of the rst so that k is negative.

quency modulation output-is` modulated-din anrplitudeby` the envelope (Ll-l-m' sin pt); `It,"thus,

conforms tothe amplitude modulated shape shown in Fig. 1A.` It may, also, be'seen that `the ratio ofthe desired signal to the undesired amp'litude modulation rinterference is equal to fthe ratio 'between the rst and second termsin pafrentheses in (5).` This ratios Il/m sin pt. The

signalto-interference ratio is thus inversely proportional to the Amodulation 'factor of the Lamplitude modulation, and, therefore, increases as the amplitude modulationfactor decreases. i

The amplitude modulation noutput 'from the 'balanced detectorsis the'result of additive combination `oi' the detected outputs.v "It is,` there- 'fforeyftlie 4sum of "(3). and (4), or: M

` Lin which the direct current `term has beenf'neglected. It is apparent that the amplitude mo'cl- `ulati'ori output is la true rendition of thattoutput by (7) vijs 4applied' as modulation, withf'the wave given by (6) 4as "the carrier. "Thus, 2(6) `is given the amplitude envelope f'(1-1m rsin pt). `The levels `are adjusted "sor that 4the modulationifa'ctor of the resulting modulation is equal to that "of the original amplitude modulation on the incoming Wave so 'that EK is ,made equal 'to unity.4

Multiplying this envelopeby (6) to obtain the resulting output, lgives:

" -aEKkEm-w ,S1112 po sin qt (s) From a comparison `of Equations (6) land (8l),

it can 'be seen that the result of the inverse modulation is to square 'the `term `m sin 'pt 'in the envelope 'produced by the amplitude ,modulation The new signal-izo-interference `ratio is hence 17m"l 'sin2 pt.` The ratio `is, therefore, increased since the `quantity `m `is less than unity'. Thus, if m is 01.5, the Signal-to-interference ratio would be 2f() Without inverse modulation, but would `be 4.0 (with inverse modulation. If m is 0.25, the ratio is increased from 4.0fto 16.

` It is appa-rent that the `improvement in signaltio-interference ratio, expressed as a number larger `than unity; is equal `to the ratio 4of m sin :ot to m2 sin2 pt, or l/m sin pt. The improvenient is,` therefore, Vinversely proportional tom, and consequently rises rapidly as m -is made smaller.

The output of the second This processof inverse modulation may be carried to an unlimited extent depending upon the degree of `amplitudemodulation reduction des'ired.` Thus, a second stage of modulation may 'follow this first fone.` The second stage would be fed by the output of the rst one which is the wave given by "(85. "The lamplitude `modulation output which is tomodulate this second stage should be passed through a square-law device before feeding lto the second modulator. This causesV the modulating wave lto have the ampli- 'tude 'mzi's'in'e'pt so that theenvelope` applied by the `'sec-ond modulator 'is (l-l-m2 sin2 pt). [The second modulator outputis', therefore:

g4-zomaar (1 -.m2 sin? pt) (i4-m2 sin2 pt) sin qt (9.)

or, Y i i `e=-2lf7`lflclla (l-m4l sin4 pti) sin qt ('10) The resulting envelope is thus proportional to "the iourth 'power `of the modulation factor, m. This increases the is aisecond 'modulator tube which is fed by the loutput voltage of the Iirst modulator tube 4 `as a source of' voltage to be modulated. The modu- 1 lationlvoltage itself `is taken from the detected voltage across potentiometer resistor `5, and

undisturbed' by' the` 'presence of the frequency .modulation` which 'has been' balanced out'.

'In the inverse modulation method? oftliisi'invention the amplitude modulation outputrgiven the AMfvoltage "is-fed to the tube I'B' of the squarelaw device which is shown in the dotted box I3'.

`'I-ube I6 'nasa highnegative bias developed across the `self-"bias resistor 116'" so that the tube is operated near plate current cut-off, and, `there `improvement factor `to lfm1 sin pt. Hence, the improvement for' the 4caseof 50 percent'modulation is 8:1, for 25 per-` vice.

Aacross resistor I I. The latter israrranged inthe plate circuit of tube I6. The voltage across resistor, I'I isapplied to lgrid I4 through a path `which includes the coupling condenser II' of the potentiometer resistor I2. The lower end of the latter is connected to the grounded lend of the grid biasing resistor of tube I I. Thepotentiome- `of proper polarity is fed to the modulating grid I4. `Potentiometer yI2 adjusts theY degree of modulation of tube I I. The inversely modulated output of tube II is developed across resistor I3, and the output voltage may be treated in the 4same manner as in the caseof the voltage developed across resistor I in thevcase of the rst modulator tube.

In other words,l a tube I9, which is similar to the tube 7., is employed7 and the grid 22 of one of its triode sections may have applied to it by means of potentiometer 2D the voltage developed across lresistor I3. The common cathode lead koi tube I9 is connected to the grounded end of vresistor 20 through the resistor 24. 23 of the second triode section of tube I9 is con- `nected to the potentiometer resister 2I which The grid has applied to it the voltage developed across f resistor II of the square-*law tube I6. `The final output voltage is developed across resistor 9' which is arranged in the common` plate circuit of` tube I9. It will, therefore, be seen that the amplified amplitudel modulation component developed across resistor AI3 is eliminated in the same manner as was done in the case of the rstmodulator tube 4.

It is apparent that more stages of'm'odulation.

may be added, if it is desired to reduce the amplitude modulationfcomponent still further. third stage of inverse modulation will require a modulating voltage raised `to the fourth power,

and this may be done'by applying ya second square-law network to the output of device I3".

f The resulting improvement factor for this system vwill be yinversely proportional to the seventh power of the modulation factor m. While multiple stages of modulation may be used to reduce thel amplitude modulation to extremely low values, for most purposes the use of a single stage will be sufcient. The reason for this is that for signalswhich are normally used the percentage vof amplitude modulation effected by the noise is relatively low, so that the system is operating in a condition which produces a high degree of improvement by the inverse modula tion... l

In, Fig.V 4 there is shown another arrangement `for securing a single stage of inverse modulation.

However/in this case the balancing tube 'I of Fig. 2 is dispensed with. This is accomplished 'by employing an adjustable connection between the junction of resistors Ri-Rz and resistor R4. It will be observed that an adjustable tap A5I! is `provided between the last-named junction point and resistor R4. 'f'tap 5|'.is provided betweenthe left-handl ter- 'mi'nal of condenser C4 and resistor R1. The cir- Furthermore, anl adjustable plitude modulation whichis not balanced out.l In other words, the balanced detector is: ade` cuit `'arrangement of Fig. 4 is similar to that shown in Fig. 2, with the exception thattube 1- justed so that it does not completely balance out the amplitude modulation. This applies a` component of amplitude modulation to the third grid 4" of tube4 which cancels that amplified 'by the first grid 4.

amplification of the amplitude modulationrby i In this way the undesired the rst grid is eliminated. When this `off-balance of the detected FM output is effected by adjusted tap 5U there is also produced an off-balance of the detected `AM output. This must not take place since it would then allow the detected FM components toappear in the detected AM output. Hence, a pure AM signal would not be available for the inverse modulation process. In order to compensate for this effect, potentiometer `tap 5I is adjusted in the same manner as tap 50. This brings the .amplitude modulation component back into balance, and thereby eliminates/the FM component from the detected AM output.

`'radio frequency instead ofat audio frequency.

In Fig.` 5 there is shown an arrangementfor utilizing a single stage of inverse modulation, but modulation being accomplished at radio frequency so that there is no amplication of the detected AM Voltage by the modulatortube. In the arrangement of Fig. 5 the demodulator diodes 2 and 3 are shown coupled to a priorLF. amplier by means of the discriminator network shown in Fig. 2. The detected FM voltage is ap-v plied to the grid 4 of modulator tube 4 through the potentiometer resistor 6. The AM voltage is taken oil from the potentiometer resistor" 5.

AThe grid 4'." of modulator tube `I has applied to it unmodulated radio frequency oscillations from the oscillator 33. The latter includes a tank circuit 33' whichA couples the plate and grid circuit of the oscillator tube lin well known `fashion.

The tank circuit33 is adjusted to a predeter mined radio frequency value. The radio frequency oscillations aretransmitted through coupling condenser 60 to the grid 4".

dyne receiver. If this is done provision must be made in circuits 30 and 29 to pass the full range of'tuning of the oscillator. i This could be done by damping circuits 30 and 29 with proper shunt resistance. I

The tuned circuit 30 is arranged in the plate I circuit of tube 4 and is fixedly resonated to the .FM Wave voltage applied tovgridA'..

frequency of oscillation of oscillator 33.

The voltage developed across circuit 30 Iis derived -from the voltage produced by oscillator 33 `with an amplitude envelope consisting of the detected In. other words, at modulatortube 4 the .outputener'g'y of The oscillator 33 may be the'local oscillator of the superheteroj No. 275,151, sied May 23, 1939.

ostillatm,` 33 functions, as a carrier which is modu-` lated bythe detected `voltage `applied 'to grid 41g..

` TheFM Wai/e voltage developedfacross circuit 3.0 is applied tothe grid l ofthe inyerse modulater tube., 2,5'. `The rst `grid 62 of tube l2'5 has applied to. it, the AM' voltage developed `across resistor `El'and transmitted togriclJ t? by lead '63. The plate circuit of inversemodulatorA '2'5 innatedito" the frequencyflofoscillator 33. At tube 25' the. frequency modulatedA carrier voltage ap` plied@ to grid' 6.I1i`s. modulated with an "amplitude modulation` voltage applied to grid- 62 which is' thedesiredFli/l. Wave form. andwith the, en-

velope substantially reduced. This resultantllvl carrier envelopeis then detected by diode rectiiler 26 to V,obtain the desired modulation Voltage Which Was'originally. applied to the carrier at the transmitter. The `resistor 21 and' condenserw `218 provide a time constantnetwork which functions to eliminate `the radio frequency" carrier, Which Wasproduced' at oscillator 33,. from the output telminal'sofrectier 26.

The method" employed inthe present invention 1' may be applied to the reception ofphase modulatedicarrier wayes (PM). In phase modulation reception the presence of'amplitud'e modulation produces the same `kind of modulation of the output as in the case, ofjFM waves. Consequent- 1y,` the same type of' circuits may be used for reducing, the undesired component: Referring to Eig,[.`6`which shows, a receiving'system for PM Wailea, the tube 4t' is` an I; F. amplieron which are` impressed PM` Waves reduced to the. I. F; value. The I. Fl transformer All; Whose primary and' secondary circuits are each i'lXedly reso natedto the operating I. F. value, i`s adjusted' to DE. of the bandV pass type.

j'The coilof; the tuned secondary circuit isv midtapped to ground for the purpose of' .providing neutralizingV voltageior `the piezo-,electric .cryst'alll, `The crystal'. is included in a crystal filter networlnj and provid-es a phase modulation discri'minator `ofj theoverand under-neutralized type.. as described in my. U". S. `Patient 222041575. The 'neutralizing condensers; li areladjusted so that an over-neutralized' characteristic is fed to one of; the two diodesv 4'3, 44;,Whil'e an underneutralized characteristic. is providedwith re-` spectto the. other oi` the diodes;` This produces abalanced phase 'modulation detector which `delivers AFC (automatic frequency control) voltageat the terminal designated AFC; Detected PMjvoltage is, developed across the potentiometer resistor V49;

' Tube 4T is a 'separate multigrid'fdetector, for amplitude modulationidetection. For this de# tection,"`1tered carrieris obtained by combining the overand under-neutralized outputs by means of condensers 80 and 8. This combination `produces a neutralized filter output Which is fed to the grid of limiter tube 58 the construction of the latter is disclosed in application Serial The output of the limiter tube is fed to one of the grids of detector tube 41, while the other grid is fedby the unfiltered signal energy from the secondary of transformer 4I.

This combination of filtered fed tc. the third grid. of modulator' tube' 45801 The" detected4` voltage mayl be taken from the l Thed modulation occurring `at tube 454 is adjusted' to have* the right pcil'aiitydgoI oppose the'un'desired: AM-" envelopev which exists on thedetected PM" voltage envelope The`f polarityl may beadusted" by "reversingA the'l phase orY either" the carrier orunfiltered?"signa-lsedtb` the` grids off detectortube l ofradio-frequency `voltages Whosel relative ain-- 4111? Theil tube ll operatesiV as" disclosed in Fig. 2" forjeXa-mple, to' balance'eut-theamplification of the detected M'component in the output circuit oithefinverse modulator'tubei". p While-If havel indicated` andldescribedi severai sys-temsl for carrying my inventioninto" effect; it*

will: be apparent to `one skilledl in the art that my invention is bync -nieans-` limited-to the parfti'cula-ru `orgamica-tions` shownf and; described; but that many' modiiicationsmaybe made Without dep`artingfrom` the scopecfmy invention, as` set forth theeappendedi'claims:

Whatclaim is:` I1.' In combinationWVith ampaizl` of* rectiilers;` a source of timing modulated carrier waves subjectwttramplitudev` 'modulation-1 c the carrier Waves; means forderi'vingfromtheWaves a pair plitudes are equalfat the meanfrequency of the; modulated? carrier waves" andvary' relativelyv in accordance with"` the` timingfmdul'ation of the carrier Waves, means? separately applying `each radio- `frequency voltage-ltd :irrespective one of said rectiiiers; means` for di-:rivingfromsaid rectiiers' in oppositixlirr af modulation toltage, compxi'sin'gY :Qu-componenti` corresponding "to said tim- `ing;modulationl andi 'a component corresponding toL saidk amplitude modulation;` means' for deri'ving from said' rectifl'ers` in'. aiding" phase `alnoduf lation-j Voitage correslzioriding;` essentially t'o said amplitude modulation,P andmeans for combinin'efsaid1 two madman-on voltages in such ren.`

non that said' ampiitude moduiauen component ofjthje'rst .modulationnyoltage is' in opposition to the second' modulaticnvoltage thereby to pro.- vidje a resultant timing modulation-voltagewith amplitude modulation component substantially reduced.` l

2. In, a..l receiver ofi timihgrmodulated carrier, waves,Y a. single means for. detecting, the. waves. to provide aflrst modulation voltage corresponding to the. timing-modulation andljundesiredi ampli` tudemodulatilon, the carrier wavesgsaid'sin.-

gle means providing a. second"4 .modulation voltase` correspondan te saiciundesirea, amplitude modulation,y'` ,means for. utilizing the. irst` modu.- lation voltage, and additional means subsequent to said detecting means for substantially reducing the effects of undesired amplitude modulation which may exist on said waves applied to said single detecting means, said additional means comprising modulator means for combin-` ing said rst modulation voltage from said detecting means with said second modulation voltage in such a sense to `produce said reduction, and an auxiliary device for` further combining said second modulation voltage with the resultant of said last` combined modulation voltages to balance out any residual amplitude modulation which may exist on said resultant.

l3.`In1a receiver of timing-modulated carrier waves,'means for detecting the waves to provide modulation voltage corresponding to the timingmodulation on the carrier waves, means for utilizing the` modulation voltage, and additional means subsequent to said detecting means for substantially lreducing the effects of undesired amplitude modulation which may exist on said waves applied to lsaid detecting means, said additional means comprisingmeans for deriving a second modulation voltage from said waves which corvrespcvmds to said undesired amplitude modulation, and means for combining said first modulation voltage from` said detecting meansV with said second modulation voltage in such a sense as to producesaid reduction, a square-law device,v means to apply said second modulation voltage te said square-law device, means to combine the resultant output of said combining kmeans* with the output voltage of said squarelaw device in a sensel to still further reducesaid undesired amplitude modulation.

f4. lIn a frequency modulated carrier wave system, a common detector for such waves consisting cf a pair of `rectifier networks connected in opposition to produce a first modulation voltage corresponding to boththe frequency modulationcomponent and an amplitude modulation component of the modulated waves and connected in aiding phase to produce a second modulationy voltage corresponding solely to the amplitude modulation componentof said waves, Aand means for inversely modulating said first modulation voltage with said second voltage thereby to? cancel out modulation effects due to thepsaid amplitude modulation component.

` L5. In a frequency modulated carrier wave sys- .temja common detector for such waves consistingof'a pairoil rectifier networks connected in opposition to produce a rstmodulation voltage corresponding to the `frequencymodulation component of.v the'modulated waves and connected in aiding phaseV to producea second voltage corresponding to 'an amplitude modulation component of said' waves,l and means for inversely modulating said 4firstl modulation Voltage with said secondlvolta'ge tocancel out any amplitude modulation eifects'on the carrier, said last means comprising a tube having at least two Vspaced control elements, and means for applying said two'ivoltages separately to respective ones of said two 'control elements.

u v6. A method of reducing undesired amplitude modulationexistin'g on frequency modulated carrier waves which includes deriving a first voltage from'the waves corresponding to the frequency modulation', modulating a high frequency carrier of constant amplitude with said first volt-4 age to'produce a modulated carrier whose amplitude is varied in accordance with the rst voltaga[deriving` a, second voltage from the said lio waves corresponding to said undesiredamplitude, modulation, inversely Amodulating the modulated' carrier with said second voltage, and rectifying' the inversely modulated carrier;v

'7. In a receiver of frequency modulated carrier Wave energy, a demodulato'r circuit con; structed'and arrangedto provide a first modula-` tion voltage'consisting of a component corre-1 sponding to the frequency modulation on the car-z rier wave and a component corresponding to undesired amplitude modulation, means to pro#L vide a second modulation voltage corresponding solely to said undesired amplitude modulation; means for combining said two modulation voltages in relative phases substantially to reduce; any undesired amplitude modulation component which may exist in saidrst modulation voltage, land at least one auxiliary means for combining the` resultant output of said combining means with a predetermined portion of the second modulation voltage thereby further to reduce said undesired amplitude modulation compo-J nent. 8. In a receiver of frequency modulated carrier wave energy, a demodulator circuit con-r structed and arranged to provide a first modu- 4 lation voltage composed of one lcomponent cor-, responding to the frequency modulation on the carrier wave and a second component corre'- spending to undesired amplitude modulation, means to provide a second modulation voltage corresponding solely to said undesired amplitude modulation, means for combining said twomodu-` lation voltages in relative phases substantially to reduce any undesired amplitude modulation component which may exist in said first modulation voltage, and at least oneauxiliary means for combining the resultant output of said combining means with a predetermined portion of the second modulation voltage thereby further to reduce rsaid undesired amplitude component;A an inverse modulator following said last com-` bining means, means applying the output o-f ,the

last combining means to said modulator, vand a Asquare law device coupling an inputelectrode of the modulator to said second modulation voltage means. Y 1

`9. In combination with a rst source of voltage representative of a desired form of `modulation and an undesired different form of modulation,.

means to provide a .second voltage representa-' tive of solely said undesiredform of modulation, an inverse modulator coupled to said first source and said means and arranged to provide av re-v'. 

